Laser produced plasma EUV light source having a droplet stream produced using a modulated disturbance wave

ABSTRACT

A plasma generating system is disclosed having a source of target material droplets, e.g. tin droplets, and a laser, e.g. a pulsed CO 2  laser, producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation. For the device, the droplet source may comprise a fluid exiting an orifice and a sub-system producing a disturbance in the fluid which generates droplets having differing initial velocities causing at least some adjacent droplet pairs to coalesce together prior to reaching the irradiation region. In one implementation, the disturbance may comprise a frequency modulated disturbance waveform and in another implementation, the disturbance may comprise an amplitude modulated disturbance waveform.

The present application is related to co-pending U.S. patent applicationSer. No. 11/358,988 filed on Feb. 21, 2006, entitled LASER PRODUCEDPLASMA EUV LIGHT SOURCE WITH PRE-PULSE, co-pending U.S. patentapplication Ser. No. 11/067,124 filed on Feb. 25, 2005, entitled METHODAND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, co-pending U.S.patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, entitledLPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, co-pending U.S.patent application Ser. No. 11/358,983 filed on Feb. 21, 2006, entitledSOURCE MATERIAL DISPENSER FOR EUV LIGHT SOURCE, co-pending U.S. patentapplication Ser. No. 11/358,992 filed on Feb. 21, 2006, entitled LASERPRODUCED PLASMA EUV LIGHT SOURCE, co-pending U.S. patent applicationSer. No. 11/174,299 filed on Jun. 29, 2005, and entitled, LPP EUV LIGHTSOURCE DRIVE LASER SYSTEM, co-pending U.S. patent application Ser. No.11/406,216 filed on Apr. 17, 2006 entitled ALTERNATIVE FUELS FOR EUVLIGHT SOURCE, co-pending U.S. patent application Ser. No. 11/580,414filed on Oct. 13, 2006 entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUVLIGHT SOURCE, and co-pending U.S. patent application Ser. No. 11/644,153filed on Dec. 22, 2006 entitled, LASER PRODUCED PLASMA EUV IGHT SOURCE,co-pending U.S. patent application Ser. No. 11/505,177 filed on Aug. 16,2006, entitled EUV OPTICS, co-pending U.S. patent application Ser. No.11/452,558 filed on Jun. 14, 2006 entitled DRIVE LASER FOR EUV LIGHTSOURCE, co-pending U.S. Pat. No. 6,928,093, issued to Webb, et al. onAug. 9, 2005, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, U.S.application Ser. No. 11/394,512, filed on Mar. 31, 2006 and titledCONFOCAL PULSE STRETCHER, U.S. application Ser. No. 11/138,001 filed onMay 26, 2005 and titled SYSTEMS AND METHODS FOR IMPLEMENTING ANINTERACTION BETWEEN A LASER SHAPED AS A LINE BEAM AND A FILM DEPOSITEDON A SUBSTRATE, and U.S. application Ser. No. 10/141,216, filed on May7, 2002, now U.S. Pat. No. 6,693,939, and titled, LASER LITHOGRAPHYLIGHT SOURCE WITH BEAM DELIVERY, U.S. Pat. No. 6,625,191 issued toKnowles et al on Sep. 23, 2003 entitled VERY NARROW BAND, TWO CHAMBER,HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. application Ser. No.10/012,002, U.S. Pat. No. 6,549,551 issued to Ness et al on Apr. 15,2003 entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL, U.S.application Ser. No. 09/848,043, and U.S. Pat. No. 6,567,450 issued toMyers et al on May 20, 2003 entitled VERY NARROW BAND, TWO CHAMBER, HIGHREP RATE GAS DISCHARGE LASER SYSTEM, U.S. application Ser. No.09/943,343, co-pending U.S. patent application Ser. No. 11/509,925 filedon Aug. 25, 2006, entitled SOURCE MATERIAL COLLECTION UNIT FOR A LASERPRODUCED PLASMA EUV LIGHT SOURCE, the entire contents of each of whichare hereby incorporated by reference herein.

FIELD

The present disclosure relates to extreme ultraviolet (“EUV”) lightsources that provide EUV light from a plasma that is created from atarget material and collected and directed to an intermediate region forutilization outside of the EUV light source chamber, e.g. by alithography scanner/stepper.

BACKGROUND

Extreme ultraviolet light, e.g., electromagnetic radiation havingwavelengths of around 50 nm or less (also sometimes referred to as softx-rays), and including light at a wavelength of about 13.5 nm, can beused in photolithography processes to produce extremely small featuresin substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has at least oneelement, e.g., xenon, lithium or tin, with one or more emission line inthe EUV range. In one such method, often termed laser produced plasma(“LPP”) the required plasma can be produced by irradiating a targetmaterial having the required line-emitting element, with a laser beam.

One particular LPP technique involves irradiating a target materialdroplet with one or more pre-pulse(s) followed by a main pulse. In thisregard, CO₂ lasers may present certain advantages as a drive laserproducing “main” pulses in an LPP process. This may be especially truefor certain target materials such as molten tin droplets. For example,one advantage may include the ability to produce a relatively highconversion efficiency e.g., the ratio of output EUV in-band power todrive laser input power.

In more theoretical terms, LPP light sources generate EUV radiation bydepositing laser energy into a source element, such as xenon (Xe), tin(Sn) or lithium (Li), creating a highly ionized plasma with electrontemperatures of several 10's of eV. The energetic radiation generatedduring de-excitation and recombination of these ions is emitted from theplasma in all directions. In. one common arrangement, anear-normal-incidence mirror (often termed a “collector mirror”) ispositioned at a distance from the plasma to collect, direct (and in somearrangements, focus) the light to an intermediate location, e.g., focalpoint. The collected light may then be relayed from the intermediatelocation to a set of scanner optics and ultimately to a wafer. In morequantitative terms, one arrangement that is currently being developedwith the goal of producing about 100 W at the intermediate locationcontemplates the use of a pulsed, focused 10-12 kW CO₂ drive laser whichis synchronized with a droplet generator to sequentially irradiate about40,000-100,000 tin droplets per second. For this purpose, there is aneed to produce a stable stream of droplets at a relatively highrepetition rate (e.g. 40-100 kHz or more) and deliver the droplets to anirradiation site with high accuracy and good repeatability in terms oftiming and position (i.e. with very small “jitter”) over relatively longperiods of time.

For a typical LPP setup, target material droplets are generated and thentravel within a vacuum chamber to an irradiation site where they areirradiated, e.g. by a focused laser beam. In addition to generating EUVradiation, these plasma processes also typically generate undesirableby-products in the plasma chamber (e.g. debris) that can potentiallydamage or reduce the operational efficiency of the various plasmachamber optical elements. These debris can include high-energy ions andscattered debris from the plasma formation, e.g., atoms and/orclumps/microdroplets of source material. For this reason, it is oftendesirable to use so-called “mass limited” droplets of source material toreduce or eliminate the formation of debris. The use of “mass limited”droplets also may result in a reduction in source material consumption.Techniques to achieve a mass-limited droplet may involve diluting thesource material and/or using relatively small droplets. For example, theuse of droplets as small as 10-50 μm is currently contemplated.

In addition to their effect on optical elements in the vacuum chamber,the plasma by-products may also adversely affect the droplet(s)approaching the irradiation site (i.e. subsequent droplets in thedroplet stream). In some cases, interactions between droplets and theplasma by-products may result in a lower EUV output for these droplets.In this regard, U.S. Pat. No. 6,855,943 (hereinafter the '943 patent)which issued to Shields on Feb. 15, 2005 and is entitled “DROPLET TARGETDELIVERY METHOD FOR HIGH PULSE-RATE LASER-PLASMA EXTREME ULTRAVIOLETLIGHT SOURCE” discloses a technique in which only some of the dropletsin a droplet stream, e.g., every third droplet, is irradiated to producea pulsed EUV light output. As disclosed in the '943 patent, thenonparticipating droplets (so-called buffer droplets) advantageouslyshield the next participating droplet from the effects of the plasmagenerated at the irradiation site. However, the use of buffer dropletsmay increase source material consumption and/or vacuum chambercontamination and/or may require droplet generation at a frequency muchhigher (e.g. by a factor of two or more) than required without the useof buffer droplets. On the other hand, if the spacing between dropletscan be increased, the use of buffer droplets may be reduced oreliminated. Thus, droplet size, spacing and timing consistency (i.e.jitter) tend to be on the top of the list of factors to be consideredwhen designing a droplet generator for an LPP EUV light source.

One technique for generating droplets involves melting a targetmaterial, e.g. tin, and then forcing it under high pressure through arelative small diameter orifice, e.g. 5-30 μm. Under most conditions,naturally occurring instabilities, e.g. noise, in the stream exiting theorifice may cause the stream to break up into droplets. In order tosynchronize the droplets with optical pulses of the LPP drive laser, arepetitive disturbance with an amplitude exceeding that of the randomnoise may be applied to the continuous stream. By applying a disturbanceat the same frequency (or its higher harmonics) as the repetition rateof the pulsed laser, the droplets can be synchronized with the laserpulses. In the past, the disturbance has typically been applied to thestream by driving an electro-actuatable element (such as a piezoelectricmaterial) with a waveform of a single frequency such as a sinusoidalwaveform, triangular waveform, square waveform or their equivalent.

As used herein, the term “electro-actuatable element” and itsderivatives, means a material or structure which undergoes a dimensionalchange when subjected to a voltage, electric field, magnetic field, orcombinations thereof and includes but is not limited to piezoelectricmaterials, electrostrictive materials and magnetostrictive materials.

In general, for the application of single frequency, non-modulatedwaveform disturbances, the spacing between droplets increases as thedisturbance frequency decreases (i.e. holding other factors such aspressure and orifice diameter constant). However, as disclosed in “DropFormation From A Vibrating Orifice Generator Driven By ModulatedElectrical Signals” (G. Brenn and U. Lackermeier, Phys. Fluids 9, 3658(1997) the contents of which are incorporated by reference herein), fordisturbance frequencies below about 0.3 υ/(πd), where υ is the streamvelocity and d is the diameter of the continuous liquid stream, morethan one droplet may be generated for each disturbance period. Thus, for10 μm liquid jet at a stream velocity of about 50 m/s, the calculatedfrequency minimum below which more than one drop per period may beproduced is about 480 kHz (note: it is currently envisioned that adroplet repetition rate of 40-100 kHz and velocities of about 30-50 m/smay be desirable for LPP EUV processes). The net result is that for theapplication of single frequency, non-modulated waveform disturbances,the spacing between droplets is fundamentally limited and cannot exceedapproximately 3.337 πd. As indicated above, it may be desirable tosupply a sufficient distance between adjacent droplets in the dropletstream to reduce/eliminate the effect of the debris from the plasma onapproaching droplet(s). Moreover, because the limitation on spacing isproportional to stream diameter, and as a consequence droplet size, thislimitation can be particularly severe in applications such as LPP EUVlight sources where relatively small, mass-limited, droplets aredesirable (see discussion above).

With the above in mind, Applicants disclose a laser produced plasma, EUVlight source having a droplet stream produced using a modulateddisturbance waveform, and corresponding methods of use.

SUMMARY

In one aspect, a device is disclosed which may comprise a plasmagenerating system having a source of target material droplets, e.g. tin,and a laser, e.g. pulsed CO₂ laser, producing a beam irradiating thedroplets at an irradiation region, the plasma producing EUV radiation.For the device, the droplet source may comprise a fluid exiting anorifice and a sub-system producing a disturbance in the fluid whichgenerates droplets having differing initial velocities causing at leastsome adjacent droplet pairs to coalesce together prior to reaching theirradiation region.

For this aspect, the ratio of initial droplets to coalesced droplets maybe two, three, four or more and in some cases ten or more. In oneembodiment, the subsystem may comprise a signal generator and anelectro-actuatable element, e.g. at least one piezoelectric crystal, andin a particular embodiment, the sub-system may comprise a capillary tubeand the disturbance may be created in the fluid by vibrating, e.g.squeezing, the capillary tube. In one implementation, the disturbancemay comprise a frequency modulated disturbance waveform and in anotherimplementation, the disturbance may comprise an amplitude modulateddisturbance waveform.

In an implementation of this aspect, the disturbance may comprise acarrier wave having a carrier wave frequency and a modulation wavehaving a frequency comprising a carrier wave frequency subharmonic. In aparticular implementation of this aspect, the laser may be a pulsedlaser having a pulse repetition rate and the disturbance may comprise amodulated disturbance waveform having a modulation frequency equal tothe pulse repetition rate.

In another aspect, a device is disclosed which may include a plasmagenerating system comprising a source of target material droplets and alaser producing a beam irradiating the droplets at an irradiationregion, the plasma producing EUV radiation. For this aspect, the dropletsource may comprise a fluid exiting an orifice and a sub-systemproducing a disturbance in the fluid, the disturbance comprising atleast two characteristic frequencies.

In a further aspect, a device is disclosed which may include a means forforcing a fluid through an orifice, a means operable on the fluid togenerate a first droplet and a second droplet, the first droplet havinga different initial velocity than the second droplet causing the firstand second droplet to coalesce together prior to reaching an irradiationregion, and a means for irradiating the droplets at the irradiationregion to form a plasma. In one implementation, the means operable onthe fluid may generate a third droplet having an initial velocity tocause the first, second and third droplets to coalesce together prior toreaching the irradiation region. In one embodiment, the means operableon the fluid may comprise one electro-actuable element and in anotherembodiment, the means operable on the fluid may comprise a plurality ofelectro-actuable elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified, schematic view of a laser produced plasma EUVlight source;

FIG. 2 shows a schematic a simplified droplet source;

FIGS. 2A-2D illustrate several different techniques for coupling anelectro-actuable element with a fluid to create a disturbance in astream exiting an orifice;

FIG. 3 illustrates the pattern of droplets resulting from a singlefrequency, non-modulated disturbance waveform;

FIG. 4 illustrates the pattern of droplets resulting from a amplitudemodulated disturbance waveform;

FIG. 5 illustrates the pattern of droplets resulting from a frequencymodulated disturbance waveform;

FIG. 6 shows photographs of tin droplets obtained for a singlefrequency, non-modulated waveform disturbance and several frequencymodulated waveform disturbances;

FIG. 7 illustrates a droplet pattern achievable using a modulatedwaveform disturbance in which droplet pairs reach the irradiation regionallowing one droplet to shield subsequent droplet pairs from plasmadebris; and

FIG. 8 illustrates a droplet pattern achievable using a modulatedwaveform disturbance in which droplet pairs reach the irradiation regionwith a first droplet reflecting light into a self-directing laser systemto initiate a discharge which irradiates the second droplet to producean EUV emitting plasma.

DETAILED DESCRIPTION

With initial reference to FIG. 1 there is shown a schematic view of anEUV light source, e.g., a laser-produced-plasma, EUV light source 20according to one aspect of an embodiment. As shown in FIG. 1, anddescribed in further details below, the LPP light source 20 may includea system 22 for generating a train of light pulses and delivering thelight pulses into a chamber 26. As detailed below, each light pulse maytravel along a beam path from the system 22 and into the chamber 26 toilluminate a respective target droplet at an irradiation region 28.

Suitable lasers for use as the device 22′ shown in FIG. 1 may include apulsed laser device, e.g., a pulsed gas discharge CO₂ laser deviceproducing radiation at 9.3 μm or 10.61 μm, e.g., with DC or RFexcitation, operating at relatively high power, e.g., 10 kW or higherand high pulse repetition rate, e.g., 50 kHz or more. In one particularimplementation, the laser may be an axial-flow RF-pumped CO₂ having aMOPA configuration with multiple stages of amplification and having aseed pulse that is initiated by a Q-switched Master Oscillator (MO) withlow energy and high repetition rate, e.g., capable of 100 kHz operation.From the MO, the laser pulse may then be amplified, shaped, and focusedbefore entering the LPP chamber. Continuously pumped CO₂ amplifiers maybe used for the system 22′. For example, a suitable CO₂ laser devicehaving an oscillator and three amplifiers (O-PA1-PA2-PA3 configuration)is disclosed in co-pending U.S. patent application Ser. No. 11/174,299filed on Jun. 29, 2005, and entitled, LPP EUV LIGHT SOURCE DRIVE LASERSYSTEM, the entire contents of which have been previously incorporatedby reference herein. Alternatively, the laser may be configured as aso-called “self-targeting” laser system in which the droplet serves asone mirror of the optical cavity. In some “self-targeting” arrangements,a master oscillator may not be required. Self targeting laser systemsare disclosed and claimed in co-pending U.S. patent application Ser. No.11/580,414 filed on Oct. 13, 2006 entitled, DRIVE LASER DELIVERY SYSTEMSFOR EUV LIGHT SOURCE, the entire contents of which have been previouslyincorporated by reference herein.

Depending on the application, other types of lasers may also besuitable, e.g., an excimer or molecular fluorine laser operating at highpower and high pulse repetition rate. Examples include, a solid statelaser, e.g., having a fiber or disk shaped active media, a MOPAconfigured excimer laser system, e.g., as shown in U.S. Pat. Nos.6,625,191, 6,549,551, and 6,567,450, an excimer laser having one or morechambers, e.g., an oscillator chamber and one or more amplifyingchambers (with the amplifying chambers in parallel or in series), amaster oscillator/power oscillator (MOPO) arrangement, a poweroscillator/power amplifier (POPA) arrangement, or a solid state laserthat seeds one or more excimer or molecular fluorine amplifier oroscillator chambers, may be suitable. Other designs are possible.

As further shown in FIG. 1, the EUV light source 20 may also include atarget material delivery system 24, e.g., delivering droplets of atarget material into the interior of a chamber 26 to the irradiationregion 28 where the droplets will interact with one or more lightpulses, e.g., one or more pre-pulses and thereafter one or more mainpulses, to ultimately produce a plasma and generate an EUV emission. Thetarget material may include, but is not necessarily limited to, amaterial that includes tin, lithium, xenon or combinations thereof. TheEUV emitting element, e.g., tin, lithium, xenon, etc., may be in theform of liquid droplets and/or solid particles contained within liquiddroplets. For example, the element tin may be used as pure tin, as a tincompound, e.g., SnBr₄, SnBr₂, SnH₄, as a tin alloy, e.g., tin-galliumalloys, tin-indium alloys, tin-indium-gallium alloys, or a combinationthereof. Depending on the material used, the target material may bepresented to the irradiation region 28 at various temperatures includingroom temperature or near room temperature (e.g., tin alloys, SnBr₄) atan elevated temperature, (e.g., pure tin) or at temperatures below roomtemperature, (e.g., SnH₄), and in some cases, can be relativelyvolatile, e.g., SnBr₄. More details concerning the use of thesematerials in an LPP EUV source is provided in co-pending U.S. patentapplication Ser. No. 11/406,216 filed on Apr. 17, 2006 entitledALTERNATIVE FUELS FOR EUV LIGHT SOURCE, the contents of which have beenpreviously incorporated by reference herein.

Continuing with FIG. 1, the EUV light source 20 may also include anoptic 30, e.g., a collector mirror in the form of a truncated ellipsoidhaving, e.g., a graded multi-layer coating with alternating layers ofMolybdenum and Silicon. FIG. 1 shows that the optic 30 may be formedwith an aperture to allow the light pulses generated by the system 22 topass through and reach the irradiation region 28. As shown, the optic 30may be, e.g., an ellipsoidal mirror that has a first focus within ornear the irradiation region 28 and a second focus at a so-calledintermediate region 40 where the EUV light may be output from the EUVlight source 20 and input to a device utilizing EUV light, e.g., anintegrated circuit lithography tool (not shown). It is to be appreciatedthat other optics may be used in place of the ellipsoidal mirror forcollecting and directing light to an intermediate location forsubsequent delivery to a device utilizing EUV light, for example theoptic may be parabolic or may be configured to deliver a beam having aring-shaped cross-section to an intermediate location, see e.g.co-pending U.S. patent application Ser. No. 11/505,177 filed on Aug. 16,2006, entitled EUV OPTICS, the contents of which are hereby incorporatedby reference.

Continuing with reference to FIG. 1, the EUV light source 20 may alsoinclude an EUV controller 60, which may also include a firing controlsystem 65 for triggering one or more lamps and/or laser devices in thesystem 22 to thereby generate light pulses for delivery into the chamber26. The EUV light source 20 may also include a droplet positiondetection system which may include one or more droplet imagers 70 thatprovide an output indicative of the position of one or more droplets,e.g., relative to the irradiation region 28. The imager(s) 70 mayprovide this output to a droplet position detection feedback system 62,which can, e.g., compute a droplet position and trajectory, from which adroplet error can be computed, e.g., on a droplet by droplet basis or onaverage. The droplet error may then be provided as an input to thecontroller 60, which can, for example, provide a position, directionand/or timing correction signal to the system 22 to control a sourcetiming circuit and/or to control a beam position and shaping system,e.g., to change the location and/or focal power of the light pulsesbeing delivered to the irradiation region 28 in the chamber 26.

The EUV light source 20 may include one or more EUV metrologyinstruments for measuring various properties of the EUV light generatedby the source 20. These properties may include, for example, intensity(e.g., total intensity or intensity within a particular spectral band),spectral bandwidth, polarization, beam position, pointing, etc. For theEUV light source 20, the instrument(s) may be configured to operatewhile the downstream tool, e.g., photolithography scanner, is on-line,e.g., by sampling a portion of the EUV output, e.g., using a pickoffmirror or sampling “uncollected” EUV light, and/or may operate while thedownstream tool, e.g., photolithography scanner, is off-line, forexample, by measuring the entire EUV output of the EUV light source 20.

As further shown in FIG. 1, the EUV light source 20 may include adroplet control system 90, operable in response to a signal (which insome implementations may include the droplet error described above, orsome quantity derived therefrom) from the controller 60, to e.g., modifythe release point of the target material from a droplet source 92 and/ormodify droplet formation timing, to correct for errors in the dropletsarriving at the desired irradiation region 28 and/or synchronize thegeneration of droplets with the pulsed laser system 22.

FIG. 2 illustrates the components of a simplified droplet source 92 inschematic format. As shown there, the droplet source 92 may include areservoir 94 holding a fluid, e.g. molten tin, under pressure. Alsoshown, the reservoir 94 may be formed with an orifice 98 allowing thepressurized fluid 98 to flow through the orifice establishing acontinuous stream 100 which subsequently breaks into a plurality ofdroplets 102 a, b.

Continuing with FIG. 2, the droplet source 92 shown further includes asub-system producing a disturbance in the fluid having anelectro-actuatable element 104 that is operable coupled with the fluid98 and a signal generator 106 driving the electro-actuatable element104. FIGS. 2A-2D show various ways in which one or moreelectro-actuatable elements may be operable coupled with the fluid tocreate droplets. Beginning with FIG. 2A, an arrangement is shown inwhich the fluid is forced to flow from a reservoir 108 under pressurethrough a tube 110, e.g. capillary tube, having an inside diameterbetween about 0.5-0.8 mm, and a length of about 10 to 50 mm, creating acontinuous stream 112 exiting an orifice 114 of the tube 110 whichsubsequently breaks up into droplets 116 a,b. As shown, anelectro-actuatable element 118 may be coupled to the tube For example,an electro-actuatable element may be coupled to the tube 110 to deflectthe tube 110 and disturb the stream 112. FIG. 2B shows a similararrangement having a reservoir 120, tube 122 and a pair ofelectro-actuatable elements 124, 126, each coupled to the tube 122 todeflect the tube 122 at a respective frequency. FIG. 2C shows anothervariation in which a plate 128 is positioned in a reservoir 130 moveableto force fluid through an orifice 132 to create a stream 134 whichbreaks into droplets 136 a,b. As shown, a force may be applied to theplate 128 and one or more electro-actuatable elements 138 may be coupledto the plate to disturb the stream 134. It is to be appreciated that acapillary tube may be used with the embodiment shown in FIG. 2C. FIG. 2Dshows another variation in which a fluid is forced to flow from areservoir 140 under pressure through a tube 142 creating a continuousstream 144 exiting an orifice 146 of the tube 142 which subsequentlybreaks up into droplets 148 a,b. As shown, an electro-actuatable element150, e.g. having a ring-like shape, may be positioned around the tube142. When driven, the electro-actuatable element 142 may selectivelysqueeze the tube 142 to disturb the stream 144. It is to be appreciatedthat two or more electro-actuatable elements may be employed toselectively squeeze the tube 142 at respective frequencies.

More details regarding various droplet dispenser configurations andtheir relative advantages may be found in co-pending U.S. patentapplication Ser. No. 11/358,988 filed on Feb. 21, 2006, entitled LASERPRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, co-pending U.S. patentapplication Ser. No. 11/067,124 filed on Feb. 25, 2005, entitled METHODAND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, and co-pending U.S.patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, entitledLPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, the contents ofeach of which are hereby incorporated by reference.

FIG. 3 illustrates the pattern of droplets 200 resulting from a singlefrequency, sine wave disturbance waveform 202 (for disturbancefrequencies above about 0.3 υv/(πd)). It can be seen that each period ofthe disturbance waveform produces a droplet and the resulting dropletsare spaced by one disturbance waveform wavelength. FIG. 3 alsoillustrates that the droplets do not coalesce together, but rather, eachdroplet is established with the same initial velocity.

FIG. 4 illustrates the pattern of droplets 300 initially resulting froman amplitude modulated disturbance waveform 302, which however is unlikethe disturbance waveform 202 described above in that it is not limitedto disturbance frequencies above about 0.3 υ/(πd)). It can be seen thatthe amplitude modulated waveform disturbance 302 includes twocharacteristic frequencies, a relatively large frequency, e.g. carrierfrequency, corresponding to wavelength λ_(c), and a smaller frequency,e.g. modulation frequency, corresponding to wavelength, λ_(m). For thespecific disturbance waveform example shown in FIG. 4, the modulationfrequency is a carrier frequency subharmonic, and in particular, themodulation frequency is a third of the carrier frequency. With thiswaveform, FIG. 4 illustrates that each period of the disturbancewaveform corresponding to the carrier wavelength, λ_(c), produces adroplet and the resulting droplets are initially spaced by one carrierwavelength, λ_(c). FIG. 4 also illustrates that the droplets coalescetogether, resulting in a stream of larger droplets 304, with one largerdroplet for each period of the disturbance waveform corresponding to themodulation wavelength, λ_(m). It can also be seen that the resultingcoalesced droplets are spaced by one modulation wavelength, λ_(m).Arrows 306 a,b show the initial relative velocity components that areimparted on the droplets by the modulated waveform disturbance 302 andare responsible for the droplet coalescence.

FIG. 5 illustrates the pattern of droplets 400 initially resulting froma frequency modulated disturbance waveform 402, which, like thedisturbance waveform 302 described above, is not limited to disturbancefrequencies above about 0.3 υ/(πd). It can be seen that the frequencymodulated waveform disturbance 402 includes two characteristicfrequencies, a relatively large frequency, e.g. carrier frequency,corresponding to wavelength λ_(c), and a smaller frequency, e.g.modulation frequency, corresponding to wavelength, λ_(m). For thespecific disturbance waveform example shown in FIG. 5, the modulationfrequency is a carrier frequency subharmonic, and in particular, themodulation frequency is about a third of the carrier frequency. Withthis waveform, FIG. 5 illustrates that each period of the disturbancewaveform corresponding to the carrier wavelength, λ_(c) produces adroplet and the resulting droplets are initially spaced by one carrierwavelength, λ_(c). FIG. 5 also illustrates that the droplets coalescetogether, resulting in a stream of larger droplets 44, with one largerdroplet for each period of the disturbance waveform corresponding to themodulation wavelength, λ_(m). It can also be seen that the resultingcoalesced droplets are spaced by one modulation wavelength, λ_(m). Likethe amplitude modulated disturbance (i.e. FIG. 4), initial relativevelocity components are imparted on the droplets by the frequencymodulated waveform disturbance 402 and are responsible for the dropletcoalescence.

Although FIGS. 4 and 5 show and discuss embodiments having twocharacteristic frequencies, with FIG. 4 illustrating an amplitudemodulated disturbance having two characteristic frequencies and FIG. 5illustrating a frequency modulated disturbance having two frequencies,it is to be appreciated that more than two characteristic frequenciesmay be employed and that the modulation may be either angular modulation(i.e. frequency or phase modulation), amplitude modulation orcombinations thereof.

FIG. 6 shows photographs of tin droplets obtained using an apparatussimilar to FIG. 2D with an orifice diameter of about 70 μm, streamvelocity of ˜30 m/s, for a single frequency, non-modulated waveformdisturbance having a frequency of 100 kHz (top photo); a frequencymodulated waveform disturbance having a carrier frequency of 100 kHz anda modulating frequency of 10 kHz of a relatively strong modulation depth(second from top photo); a frequency modulated waveform disturbancehaving a carrier frequency of 100 kHz and a modulating frequency of 10kHz of a relatively weak modulation depth (third from top photo); afrequency modulated waveform disturbance having a carrier frequency of100 kHz and a modulating frequency of 15 kHz (fourth from top photo) afrequency modulated waveform disturbance having a carrier frequency of100 kHz and a modulating frequency of 20 kHz (bottom photo).

These photographs indicate that tin droplets having a diameter of about265 μm can be produced that are spaced apart by about 3.14 mm, a spacingwhich cannot be realized at this droplet size and repetition rate usinga single frequency, non-modulated waveform disturbance.

Measurements conducted using the droplet photos indicated a timingjitter of about 0.14% of a modulation period which is substantially lessthan the jitter observed under similar conditions using a singlefrequency, non-modulated waveform disturbance. This effect is achievedby averaging the individual droplets instabilities over a number ofcoalescing droplets.

FIG. 7 shows a droplet pattern 600 produced using a modulated, e.g.multiple frequency, disturbance waveform (see also FIG. 6 fourth photofrom top). As shown, at a selected distance from orifice 604. As shown,this droplet pattern in which droplet pairs reach the irradiation regionallows droplet 608 a to establish an EUV emitting plasma uponirradiation by the laser 22′ while droplet 608 b shields subsequentdroplet pair 610 from plasma debris.

FIG. 8 illustrates a droplet pattern 700 achievable using a modulatede.g. multiple frequency, disturbance waveform in which droplet pairsreach the irradiation region with a first droplet 702 a reflecting lightinto a self-directing laser system 704 to initiate a laser oscillationoutput laser beam which irradiates the second droplet 702 b to producean EUV emitting plasma.

Self-directing laser system 704 is more fully described in co-pendingU.S. patent application Ser. No. 11/580,414 filed on Oct. 13, 2006entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, see inparticular, FIG. 5, the entire contents of which were previouslyincorporated by reference. Although the following describes a lasersystem 704 corresponding to FIG. 5 of the Ser. No. 11/580,414 patentapplication, it is to be appreciated that this description is equallyapplicable to the other self-directed lasers disclosed in the Ser. No.11/580,414 patent application (i.e. FIGS. 6-16.) Continuing with FIG. 8,it can be seen that the self directing laser system 704 may include anoptical amplifier 706 a,b,c. For example, the optical amplifier 706 maybe a CW pumped, multiple chamber, CO₂ laser amplifier amplifying lightat a wavelength of 10.6 μm and having a relatively high two-pass gain(e.g. a two pass gain of about 1,000,000). As further shown, theamplifier 706 may include a chain of amplifier chambers 706 a-c,arranged in series, each chamber having its own active media andexcitation source, e.g. electrodes.

In use, the first droplet 702 a of target material is placed on atrajectory passing through or near a beam path 710 extending through theamplifier 706. Spontaneously emitted photons from the amplifier 706 maybe scattered by the droplet and some scattered photons may be placed onpath 710 where they travel though the amplifier 706. As shown, an optic708 may be positioned to receive the photons on path 710 from theamplifier 706 and direct the beam back through the amplifier 706 forsubsequent interaction with the second droplet 702 b to produce an EUVlight emitting plasma. For this arrangement, the optic 708 may be, forexample, a flat mirror, curved mirror, phase-conjugate mirror or cornerreflector. An optical element 714, e.g., lens may be positioned tocollimate light entering the amplifier 706 from the droplet and focuslight traveling from the amplifier 706 to the droplet. An optionaloptical delay 716 may be provided to establish the required time delaybetween when the first and second droplets reach the irradiation region.One advantage of using different droplets to 1) establish the opticaloscillator and 2) generate an EUV emitting plasma is that the size ofthe droplets may be independently optimized for their specific function(i.e, reflection versus plasma production).

While the particular embodiment(s) described and illustrated in thispatent application in the detail required to satisfy 35 U.S.C. §112 arefully capable of attaining one or more of the above-described purposesfor, problems to be solved by, or any other reasons for or objects ofthe embodiment(s) above described, it is to be understood by thoseskilled in the art that the above-described embodiment(s) are merelyexemplary, illustrative and representative of the subject matter whichis broadly contemplated by the present application. Reference to anelement in the following Claims in the singular is not intended to meannor shall it mean in interpreting such Claim element “one and only one”unless explicitly so stated, but rather “one or more”. All structuraland functional equivalents to any of the elements of the above-describedembodiment(s) that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present Claims. Any term usedin the Specification and/or in the Claims and expressly given a meaningin the Specification and/or Claims in the present Application shall havethat meaning, regardless of any dictionary or other commonly usedmeaning for such a term. It is not intended or necessary for a device ormethod discussed in the Specification as an embodiment to address orsolve each and every problem discussed in this Application, for it to beencompassed by the present Claims. No element, component, or method stepin the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the Claims. No claim element in the appendedClaims is to be construed under the provisions of 35 U.S.C. §112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or, in the case of a method claim, the element is recited asa “step” instead of an “act”.

1. A device comprising: a plasma generating system comprising a sourceof target material droplets and a laser producing a beam irradiating thedroplets at an irradiation region, the plasma producing EUV radiation,wherein the droplet source comprises a fluid exiting an orifice and asub-system producing a disturbance in the fluid, the disturbancesimultaneously comprising at least two characteristic frequencies.
 2. Adevice as recited in claim 1 wherein the subsystem comprises a signalgenerator and an electro-actuatable element.
 3. A device as recited inclaim 1 wherein the disturbance comprises a frequency modulateddisturbance waveform.
 4. A device as recited in claim 1 wherein thedisturbance comprises an amplitude modulated disturbance waveform.
 5. Adevice as recited in claim 1 wherein the disturbance comprises a carrierwave having a carrier wave frequency and a modulation wave having afrequency comprising a carrier wave frequency subharmonic.
 6. A deviceas recited in claim 1 wherein the laser is a pulsed laser having a pulserepetition rate and the disturbance comprises a modulated disturbancewaveform having a modulated frequency equal to said pulse repetitionrate.
 7. A device as recited in claim 1 wherein the disturbancegenerates droplets having controlled initial velocities causing at leastsome adjacent droplet pairs to coalesce together prior to reaching theirradiation region.
 8. A device as recited in claim 7 wherein a ratio ofinitial droplets to coalesced droplets is greater than two.
 9. A deviceas recited in claim 7 wherein the subsystem comprises a signal generatorand an electro-actuatable element.
 10. A device as recited in claim 7wherein the disturbance comprises a frequency modulated disturbancewaveform.
 11. A device as recited in claim 7 wherein the disturbancecomprises an amplitude modulated disturbance waveform.
 12. A device asrecited in claim 1 wherein the target material comprises tin.
 13. Adevice as recited in claim 1 wherein the sub-system comprises acapillary tube and the disturbance is created in the fluid by squeezingthe capillary tube.
 14. A device as recited in claim 1 wherein the lasercomprises a gain media and the gain media comprises CO₂.
 15. A device asrecited in claim 2 wherein the electro-actuatable element comprises atleast one piezoelectric crystal.
 16. A device as recited in claim 7wherein the disturbance comprises a carrier wave having a carrier wavefrequency and a modulation wave having a frequency comprising a carrierwave frequency subharmonic.
 17. A device as recited in claim 7 whereinthe laser is a pulsed laser having a pulse repetition rate and thedisturbance comprises a modulated disturbance waveform having amodulated frequency equal to said pulse repetition rate.